This invention relates to an electric shift apparatus especially suited for use with a motor vehicle having an automatic transmission.
Motor vehicles since their inception have required some manner of gear change mechanism to satisfy the varying torque and speed requirements encountered during the typical duty cycle of a motor vehicle. For many years these gear change mechanisms were manual in the sense that they required an operator input from a shift lever or the like to effect each desired gear change ratio. More recently, so called "automatic transmissions" have become popular in which much of the shifting is done without operator input in response to sensed speed and throttle opening parameters. These automatic transmissions typically include a mode select lever positioned on the transmission housing and movable between a plurality of selectively pivoted positions corresponding to a respective plurality of shift modes within the transmission. The mode select lever is pivotally moved between its several shift positions by a cable or linkage mechanism extending from the mode select lever to a suitable gear selector lever located in the passenger compartment of the vehicle. Various proposals have been made in the past to eliminate the mechanical interconnection between the driver operated lever and the mode select lever and provide instead an electrical signal generated by a suitable action on the part of the driver and transmitted electrically to some manner of power means arranged to move the mode select lever. None of these attempts to provide an electric shift mechanism for an automatic transmission of a motor vehicle have met with any degree of commercial success since they provided a slow or imprecise shifting action and/or have generated excessive warranty and maintenance costs.
Recently a new type of automatic transmission control has emerged. These new automatic transmission controls employ electronic control circuits to compare the desired transmission state based upon a operator input with the present transmission state from a transmission state sensor. The electronic control circuit compares the desired transmission state with the sensed current transmission state and controls a motor to shift the transmission when these differ. This system is exemplified in U.S. Pat. No. 4,709,204 entitled "Electric Shift Apparatus" issued Dec. 13, 1988, and U.S. Pat. No. 4,817,471 entitled "Electronic Control System for Control of a Manual Transmission Apparatus" issued Apr. 4, 1989, each of which is assigned to the assignee of the present application.
One problem with such a system is in diagnosis of transmission faults. Servicing the transmission is complicated by the additional layer of controls in the electronic control circuitry. It is difficult and time consuming to determine if the fault lies in the sensors, the electronic control circuit, the motor, or in the transmission itself. Thus there is a need in the art to improve the diagnostic and serviceability of such transmission controls.